Surely someone here has researched this question. If you restore / colorize / photo-art a public domain photograph, can you copyright the modified version? Does it make a difference if the photo has ever / never been published? Any place where I can read more?

What do you mean by "public domain photograph". All photos are copyrights protected, unless a written contract/agreement/statement is made in order to wave any claims under copyrights.
It's a very complex and messy law, that has many people confused. It really has to be down to an image by image assessment.

First, its over 70 years old. The photographer is dead and it has never been published. I believe that makes it public domain. If you have a photograph of a Van Gogh, you can publish it, say in a calendar for the same reason. I actually don't think that whether its been published or not has anything to do with whether something is public domain or not. That becomes an issue if its been published and later you apply for a copyright. Then, the prior publication has to have been labeled with a copyright notice according to the requirements of copyright law in order for a subsequently issued copyright to enforcable.

http://www.copyright.gov/circs/circ14.pdf